can hold water better than other soil types (Salt et al. 1995a, b; Taiz and Zeiger
2002). The entry of metals into the root system is by symplastic or apoplastic
pathways (Tandy et al. 2006; Lu et al. 2009). The symplast or active pathway, as
the name suggests, is an energy-associated process by which entry is mediated by a
specific ion carrier or channels, whereas in the passive or apoplastic pathway the
metal or its complexes enter the root through intercellular spaces.
7.2 Effect of Soil/Rhizospheric Microorganisms on Metal
Uptake
The rhizosphere soil nature and microbial communities are greatly influenced by the
rooting or root growth of the plants; it was reported that the rhizosphere microbial
population is of several orders of magnitude higher than that of nearer soil surroundings (Anderson 1997). These microbes symbiotically favor the root in metal uptake.
In a few cases, some microorganisms excrete important organic compounds for
facilitating metal bioavailability for roots and also for root absorption of essential
micronutrients such as Fe (Crowley et al. 1991) and Mn (Barber and Lee 1974), as
well as a few of the nonessential metals such as Cd (Salt et al. 1995a, b; Li et al.
2013); it sometimes alters the soil chemistry to make the metals more soluble.
7.3 Effect of Root Exudates on Metal Uptake
Root exudates have a significant role in phytoremediation. Some of the organic
chemicals released by root systems in certain stages of growth induce plants to
acclimatize to the stressed environment either by affecting rhizosphere microbial and
other plant growth (allelopathic functions) or by inactivating the metal pollutant by
root absorption, assimilation, chelation, and transformation (detoxification functions). In addition, root exudates, particularly organic acids, are able to bind metal
ions, therefore influencing metal mobility, solubility, and bioavailability from soil
(Chiang et al. 2011; Luo et al. 2014). Kim et al. (2010) suggest organic acids from
root exudates act as natural chelators to enhance the phytoextraction process; the
enhanced efficiency in translocation and bioaccumulation of Cd, Cu, and Pb is
achieved with the help of citric acid and oxalic acid from Echinochloa crus-galli.
7.4 Enzymatic Transformations
The metabolism or transformation of metals by plants after absorption via root
systems from contaminated sites is said to be a phyto-transformation. After entering
the root system, the metals are translocated and undergo different phases of
100
M. K. Awasthi et al.
2002). The entry of metals into the root system is by symplastic or apoplastic
pathways (Tandy et al. 2006; Lu et al. 2009). The symplast or active pathway, as
the name suggests, is an energy-associated process by which entry is mediated by a
specific ion carrier or channels, whereas in the passive or apoplastic pathway the
metal or its complexes enter the root through intercellular spaces.
7.2 Effect of Soil/Rhizospheric Microorganisms on Metal
Uptake
The rhizosphere soil nature and microbial communities are greatly influenced by the
rooting or root growth of the plants; it was reported that the rhizosphere microbial
population is of several orders of magnitude higher than that of nearer soil surroundings (Anderson 1997). These microbes symbiotically favor the root in metal uptake.
In a few cases, some microorganisms excrete important organic compounds for
facilitating metal bioavailability for roots and also for root absorption of essential
micronutrients such as Fe (Crowley et al. 1991) and Mn (Barber and Lee 1974), as
well as a few of the nonessential metals such as Cd (Salt et al. 1995a, b; Li et al.
2013); it sometimes alters the soil chemistry to make the metals more soluble.
7.3 Effect of Root Exudates on Metal Uptake
Root exudates have a significant role in phytoremediation. Some of the organic
chemicals released by root systems in certain stages of growth induce plants to
acclimatize to the stressed environment either by affecting rhizosphere microbial and
other plant growth (allelopathic functions) or by inactivating the metal pollutant by
root absorption, assimilation, chelation, and transformation (detoxification functions). In addition, root exudates, particularly organic acids, are able to bind metal
ions, therefore influencing metal mobility, solubility, and bioavailability from soil
(Chiang et al. 2011; Luo et al. 2014). Kim et al. (2010) suggest organic acids from
root exudates act as natural chelators to enhance the phytoextraction process; the
enhanced efficiency in translocation and bioaccumulation of Cd, Cu, and Pb is
achieved with the help of citric acid and oxalic acid from Echinochloa crus-galli.
7.4 Enzymatic Transformations
The metabolism or transformation of metals by plants after absorption via root
systems from contaminated sites is said to be a phyto-transformation. After entering
the root system, the metals are translocated and undergo different phases of
100
M. K. Awasthi et al.